1. Technical Field
This invention relates to a process for preparing low density silica particles by heating a mixture of boric acid or B.sub.2 O.sub.3 with high density silica particles at a temperature above about 450.degree. C. Silica particles so prepared individually may have internal porosity and exhibit unique properties, such as a small to negative thermal expansion coefficient when first heated after preparation. The particles are useful for a variety of applications, such as a catalyst support or in resin blends used in electronic packaging.
2. Background Information
Silica particles are well known in the art and have found a broad range of applications due to the chemical inertness of silica and its favorable dielectric properties. Common applications include use as a catalyst support, as media in liquid chromatography, and as filler in resins employed in electronic packaging.
Silica generally is prepared using a mechanical, fumed, or chemical process. Conventional mechanical processes produce solid particles having little or no internal porosity. Variations in bulk densities are attributed to particle size and how tightly the particles are packed. Conventional fumed silicas are produced by high temperature (1800.degree. C.) hydrolysis of silicon tetrachloride. This process is expensive, produces corrosive hydrogen chloride as a by-product, and the product silica has an undesirable low bulk density. Conventional chemical processes for producing xerogels and aerogels do create particles having internal porosity, but these processes are even more expensive than those for fumed silicas, and there is limited commercial capacity. The lower cost mechanical processes tend to produce particles having an angular morphology that are undesirable for applications such as resin/filler systems used in electronic packaging.
Thus, there continues to be a need for economical methods of producing silica particles having internal porosity. Also, there continues to be a need for silica particles having unique properties.